Application Components

8/12/2019 Application Components

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Application Components

Activitiesvisual user interface focused on a single thing a user

can do

Servicesno visual interfacethey run in the background

Broadcast Receiversreceive and react to broadcast

announcements

Content Providersallow data exchange between applications


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Activities

Basic component of most applications

Most applications have several activities that start each otheras needed

Each is implemented as a subclass of the base Activity class

An activitypresents a visual user interface for one focused the

user.

The visual content of the window is provided by a hierarchy ofviews.

objects derived from the base View class.


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Continue

For example, an email application might have one activitythat

shows a list of new emails, another activity to compose an

email, and another activity for reading emails.

Although the activities work together to form a cohesive user

experience in the email application, each one is independent of

the others


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Activities and Tasks

As, we know An activitypresents a visual user interface

Task is what the user experiences as an application.

Task is a group of related activities, arranged in a stack.

The activity at the top of the stack is one that's currentlyrunning.

All the activities in a task move together as a unit.


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Activity Lifecycle

An activity has essentially three states

Active or Running

Paused

Stopped

Activity Lifetime

Entire Lifetime [onCreate()to onDestroy()]

Visible Lifetime [onStart()to onStop()]

Foreground Lifetime [onResume()to onPause()]

An implementation of any activity lifecycle method shouldalways first call the superclassversion


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Activity Lifecycle

in detail

ResumedThe activity is in the foreground of the screen and has user

focus

Paused

Another activity is in the foreground and has focus, but thisone is still visible

Stopped

The activity is completely replaced by another activity(the activity is now in the "background"). A stoppedactivity is also still alive

It can be killed by the system when memory is needed


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Services

A Service does not have a visual interface and runs in the

background.

Each service extends the Servicebase class.

It's possible to connect to an ongoing service and communicate

it through the interface exposed by that service.

Services run in the main thread of the application process.


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Service Lifecycle

A service can be used in two ways

startService()

stopService()

bindService()

unbindService() Service Lifetime

Entire Lifetime [onCreate()to onDestroy()]

Active Lifetime [onStart()]

The onCreate() and onDestroy() methods are called for all services.

onStart()is called only for services started by startService().


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Services

Runs in the background indefinitely

Examples

Network DownloadsPlaying Music

You can bind to a an existing service and control its operation


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Broadcast Receivers

A broadcast receiveris a component that responds to system-wide broadcastannouncements. Many broadcasts originate from the system

Receive and react to broadcast announcements

Extend the class BroadcastReceiver

Examples of broadcasts:

- Low battery

- power connected

- shutdown

- timezonechanged, etc.

- Other applications can initiate broadcasts

Many broadcasts originate in system code.

Broadcast receivers do not display a user interface but they can start an activity oralert user.


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Content Providers

A content providermanages a shared set of application data.

You can store the data in the file system, an SQLite database,

on the web, or any other persistent storage location your

application can access.

For example, the Android system provides a content provider

that manages the user's contact information.


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Content Providers

Makes some of the application data available to otherapplications

Its the only way to transfer data between applications in

Android (no shared files, shared memory, pipes, etc.)

Extends the class ContentProvider;

Other applications use a ContentResolverobject to access thedata provided via a ContentProvider


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Content Providers

All content providers extends the ContentProviderbase class.

Content Providers are accessed through

ContentResolverobject.

Content Providers and Content Resolvers enable inter-process

communication (IPC)


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Intents Three of the four component typesactivities, services, and

broadcast receiversare activated by an asynchronousmessage called an intent.

Intents bind individual components to each other at runtime

An intent is an Intentobject with a message content.

An intent is an object of Intent class that holds the contentof the message.

Activities, services and broadcast receivers are started byintents.


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ContentProvidersare started by ContentResolvers:

An activityis started by Context.startActivity(Intent intent)or

Activity.startActivityForResult(Intent intent,intRequestCode)

A serviceis started by Context.startService(Intent service)

An application can initiate a broadcastby using an Intent inany of

Context.sendBroadcast(Intent intent)

Context.sendOrderedBroadcast()

Context.sendStickyBroadcast()


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Intents

Intents are Asynchronous messages used to convey a requestor message.

Intent can contain

Component name

Action

Data

Category

Flags


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Shutting down components

Activities

Can terminate itself via finish();

Can terminate other activities it started via finishActivity();

ServicesCan terminate via stopSelf(); or Context.stopService();

Content Providers

Are only active when responding to ContentResolvers

Broadcast Receivers

Are only active when responding to broadcasts


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The Manifest File

Before the Android system can start an application component,

the system must know that the component exists by reading

the application's AndroidManifest.xml file

Your application must declare all its components in this file,

which must be at the root of the application project directory.


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Explanation

element

android:iconattribute points to resources for an icon that

identifies the application

element

android:nameattribute specifies the fully qualified class

name of the Activitysubclass and the android:labelattributes

specifies a string to use as the user-visible label for the

activity.
http://developer.android.com/guide/topics/manifest/application-element.htmlhttp://developer.android.com/guide/topics/manifest/activity-element.htmlhttp://developer.android.com/reference/android/app/Activity.htmlhttp://developer.android.com/reference/android/app/Activity.htmlhttp://developer.android.com/guide/topics/manifest/activity-element.htmlhttp://developer.android.com/guide/topics/manifest/application-element.html


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Declare All Application Components

elements for activities

elements for services

elements for broadcast receivers

elements for content providers

Note :

Activities, services, and content providers that you include inyour source but do not declare in the manifest are not visible to

the system and, consequently, can never run.
http://developer.android.com/guide/topics/manifest/activity-element.htmlhttp://developer.android.com/guide/topics/manifest/service-element.htmlhttp://developer.android.com/guide/topics/manifest/receiver-element.htmlhttp://developer.android.com/guide/topics/manifest/provider-element.htmlhttp://developer.android.com/guide/topics/manifest/provider-element.htmlhttp://developer.android.com/guide/topics/manifest/receiver-element.htmlhttp://developer.android.com/guide/topics/manifest/service-element.htmlhttp://developer.android.com/guide/topics/manifest/activity-element.html

Documents

Application Components